Modern automobile engines have become more complex in recent years. In particular, automobile engines can have either a conventional ignition system or a non-distributor ignition system. In either case, if the engine is not operating properly it is necessary to evaluate whether the spark plugs of an engine are firing properly.
Most conventional ignition systems generate a high voltage negative pulse to the spark plug. Since electrons are negative, it is easier to repel them from the centre electrode to cause a spark. Non-distributor type designs, i.e. electronic ignition systems, use a high voltage positive pulse to cause the spark plug to fire.
When an ignition coil is pulsed, the secondary voltage will rise with respect to the polarity of the system. The spark will not disturb this rise in voltage until the spark potential has been reached. At this point, the spark plug undergoes a step change in terminal resistance suddenly presenting a low resistance path to ground. This will cause the energy in the coil secondary to discharge and rapidly collapse the voltage. The energy will jump across the gap causing a spark which ignites the fuel-air mixture. The effect of the spark is the generation of an initial high amplitude, high frequency, counter-voltage spark burst.
It is well known that electromagnetic emissions are generated by the ignition wires during normal operation of an engine. It is also well known that by sampling the electromagnetic emissions, ignition information can be obtained for evaluating the operation of the engine. Prior art devices are either triggered by the electromagnetic emissions or measure the amplitude of the emissions to provide spark information.
Many prior art devices merely sense the level or amplitude of high voltage along the ignition wires. The basic operating theory of these devices is that the coil voltage will only climb as high as required to cause the spark plug to fire. When the plug does fire, coil energy is released, clamping any further voltage rise. As engine speed increases, the corresponding increase in cylinder compression will require more voltage for the spark plug to fire.
Neon type testers use this theory by relating the brightness of the neon tube directly to voltage in the ignition wire. The neon tube will appear brighter with increases in engine speed. Oscilloscopes will visually display on a screen the higher voltage required for the spark plug to fire. Both of these devices translate the signal level amplitude of the ignition wires and correlate the amplitude to the potential for a spark.
In the case of the neon type tester, a certain degree of skill and training is required in order to properly interpret the brightness of the neon light as it relates to the spark condition of the ignition system. For the do-it-yourself mechanic, such device would be generally unsuitable without practice. Further, the brightness of such devices is poor making these devices unsuitable for outdoor use. Oscilloscopes are generally too expensive for the do-it-yourself mechanic.
The prior art devices generally measure or detect only an increase in voltage for a sparking potential of the ignition system. These devices are generally not suited for all conditions of spark. Merely because the ignition system has sufficient voltage to generate a spark does not necessarily mean that a spark will in fact occur.
Other devices attempt to determine whether a spark has occurred by measuring or detecting a current in the ignition wire. If the spark plug is operating or firing in a normal manner, there will be a pulsating current flow through the associated ignition wire. By detecting this current, the device indicates a spark condition.
In normal operation of an engine, these devices will detect a spark. However, this type of device will not properly diagnose a condition where the spark plug is wet. Such condition occurs when the engine is choked during start-up. In this case, the voltage increases, yet current will flow through the conductive fuel mixture thereby indicating a spark condition even though the spark plug has not fired.